1. Field of the Invention
The present invention relates to a substrate for a liquid crystal display and a liquid crystal display utilizing the same and, more particularly, to a substrate for a liquid crystal display in the VA (vertically aligned) mode in which a liquid crystal having negative dielectric anisotropy is vertically aligned, IPS (in-plane switching) mode in which a transverse electric field is applied to a horizontally aligned liquid crystal having positive dielectric anisotropy, or the like, the invention further relating to a liquid crystal display utilizing the same.
The present invention also relates to a liquid crystal display in which a liquid crystal layer including polymeric components (a monomer and an oligomer) that are optically or thermally polymerized are sealed between substrates and in which the alignment of the liquid crystal is fixed by polymerizing the polymeric components while adjusting a voltage applied to the liquid crystal layer (the applied voltage may be 0 (zero) and, hereinafter, this operation may be simply expressed as “while applying a voltages” depending on situations), the invention also relating to a substrate used for such a liquid crystal display.
2. Description of the Related Art
Multi-domain vertical alignment mode liquid crystal displays(hereinafter simply referred to as “MVA-LCDs”) are known in which a liquid crystal having negative dielectric anisotropy is vertically aligned and in which banks (linear protrusions) on the substrate or cutouts (slits) in electrodes are provided as alignment regulating structures. Since alignment regulating structures are provided, the liquid crystal can be controlled such that it is aligned in a plurality of aligning directions when a voltage is applied without any need for a rubbing process on an alignment film. Such MVA-LCDs have better viewing angle characteristics compared to conventional TN (twisted nematic) mode LCDs.
However, conventional MVA-LCDs have a problem in that they appear dark when displaying white because of low luminance. This is primarily attributable to the fact that transmittance decreases to result in dark appearance when white is displayed because dark lines appear above protrusions or slits which serve as boundaries for alignment separation. Although this problem can be mitigated by keeping sufficiently large intervals between the protrusions or slits, since this results in a reduction in the number of the protrusions or slits which are alignment regulating structures, a longer time will be required to fix the alignment of the liquid crystal after applying a predetermined voltage to the same, which results in the problem of a low response speed.
In order to mitigate this problem and to provide MVA-LCDs having high luminance and capable of high speed response, the use of a polymer-fixing method has been proposed. According to the polymer-fixing method, a liquid crystal compound obtained by mixing polymeric components such as a monomer and an oligomer (hereinafter simply represented by “monomer”) in a liquid crystal is sealed between substrates. The monomer is polymerized with liquid crystal molecules tilted by applying a voltage between the substrates. As a result, a liquid crystal layer tilted at a predetermined pre-tilt angle is obtained even after terminating the application of the voltage, which makes it possible to fix the alignment of the liquid crystal. Referring to the monomer, a material that is polymerized by heat or light (ultraviolet light) is chosen.
However, the polymer-fixing method has some problems associated with irregularities in display when an image is displayed on an LCD thus completed. The first problem is that irregularities are caused in the display of an image on the completed LCD by abnormality in the alignment of the liquid crystal that locally occurs when the liquid crystal is driven to polymerize the monomer.
IPS mode liquid crystal displays (hereinafter simply referred to as “IPS-LCDs”) in which a horizontal field is applied to a horizontally aligned liquid crystal having positive dielectric anisotropy have preferable viewing angle characteristics similarly to MVA-LCDs. However, since liquid crystal molecules are switched in a horizontal plane with comb-shaped electrodes in an IPS-LCD, there is a need for a back-light unit having high optical intensity because of a significant reduction in the aperture ratio of the pixels attributable to the comb-shaped electrodes.
The panel of an MVA-LCD has optical transmittance lower than that of a TN mode LCD, although it is subjected to a less significant reduction in the substantial aperture ratio of the pixels attributable to the protrusions or slits compared to that in an IPS-LCD attributable to the comb-shaped electrodes. For such reasons, presently, neither MVA-LCD nor IPS-LCD is used in substantially notebook type personal computer for which low power consumption is a must.
In a current MVA-LCD, in order to tilt liquid crystal molecules in four directions when a voltage is applied to achieve a wide viewing angle, a multiplicity of linear protrusions or slits that are linear cutouts in a part of a pixel electrode are provided in a pixel in a complicated configuration. This reduces the optical transmittance of the pixel.
A description will now be made on an alignment regulating operation in a case wherein large intervals are kept between adjoining linear protrusions with a simple configuration in order to mitigate the problem. FIGS. 14A and 14B show an MVA-LCD having two separately aligned regions. FIG. 14A shows a pixel 2 of the MVA-LCD as viewed in the normal direction of the substrate surfaces. FIG. 14B shows a section of the MVA-LCD shown in FIG. 14A taken in parallel with drain bus lines 6. FIG. 14A shows three pixels 2 corrected to one gate bus line 4. As shown in FIGS. 14A and 14B, two linear protrusions 68 extending in parallel with the gate bus line 4 are formed in the vicinity of both ends of pixel electrodes 3 located on a side of the gate bus line 4. A linear protrusion 66 extending in parallel with the gate bus line 4 is formed in a region of a common electrode on an opposite substrate, the region including central regions of the pixels. Referring to an array substrate, an insulation film (gate insulation film) 23 is formed on a glass substrate 20 and the gate bus line 4, and an insulation film 22 is formed on the same.
In this configuration, when a voltage is applied between the pixel electrodes 3 and the common electrode 26 to change the distribution of an electric field in a liquid crystal layer 24, liquid crystal molecules 24a having negative dielectric anisotropy are tilted in two directions. Specifically, the liquid crystal molecules 24a are tilted in directions from the linear protrusions 68 on both sides of the pixels 2 on a side of the gate bus line 4 to the linear protrusion 66 on the opposite substrate. As a result, a multi-domain is formed which is divided into two parts, i.e., upper and lower parts. In the MVA mode, the tilting direction of the liquid crystal molecules 24a is sequentially determined by an electric field generated by the linear protrusions (or slits) starting with molecules located in the vicinity of the linear protrusions 66 and 68 (or in the vicinity of the slits). Therefore, when the intervals between the linear protrusions (or slits) are very large as shown in FIGS. 14A and 14B, the response of the liquid crystal molecules to the application of a voltage becomes very slow because the propagation of the tilt of the liquid crystal molecules 24a takes time.
A possible solution to this is to use the polymer-fixing method in which a liquid crystal layer 24 including a monomer that can be polymerized instead of the conventional liquid crystal material is employed. According to the polymer-fixing method, the monomer is polymerized with a voltage applied to the liquid crystal layer 24, and the resultant polymer is cause to memorize the tilting direction of the liquid crystal molecules 24a. 
However, when a voltage is applied to the liquid crystal layer 24 in the structure shown in FIGS. 14A and 14B, liquid crystal molecules 24a in the vicinity of the drain bus lines 6 are tilted in a direction that is 90 deg. different from the intended tilting direction because of electric fields generated at the edges of the pixel electrodes 3 in the vicinity of the drain bus lines 6. As a result, even if the polymer-fixing method is adopted, large dark parts X1 extending along the drain bus line 6 outside a black matrix BM will be recognized at each of the display pixels 2 as shown in FIG. 15 that is a microscopic view of the MVA-LCD in the normal direction of the substrate surfaces.
In order to solve this, in a previous application filed by the present applicant (Japanese patent application No. 2001-264117 filed on Aug. 31, 2001), a proposal was made in which pixel electrodes 3 on an array substrate having TFTs 16 formed thereon are stripe-shaped electrodes in a line-and-space pattern. By way of example, FIG. 16 shows an embodiment in which a pixel 2 of an MVA-LCD is viewed in the normal direction of the substrate surfaces. As shown in FIG. 16, a pixel electrode 3 has stripe-shaped electrodes 8 and spaces 10 formed in a line-and-space pattern in parallel with a drain bus line 6.
In general, an alignment regulating force provided by an alignment film acts only on liquid crystal molecules 24a in contact with the alignment film and does not act on liquid crystal molecules in the middle of the device in the direction of the cell gap. Therefore, the aligning direction of liquid crystal molecules 24a in the middle of the device in the direction of the cell gap is significantly affected and disturbed by electric fields generated at an edge of a pixel. In the case of the pixel electrode 3 having the stripe-shaped electrodes 8 and spaces 10 in parallel with the drain bus line 6, liquid crystal molecules 24a are tilted in parallel with the stripe-shaped electrodes 8 and spaces 10 when a voltage is applied. Further, since the tilting direction of all liquid crystal molecules 24a is determined by the stripe-shaped electrodes 8 and spaces 10, the influence of a transverse electric field generated at an edge of the pixel can be minimized.
The liquid crystal display proposed in the above-mentioned application and a method of manufacturing the same will now be specifically described. FIG. 16 shows the pixel 2 of the MVA-LCD according to the proposal as viewed in the normal direction of the substrate surfaces, and FIG. 17 shows a sectional configuration taken along the line D—D in FIG. 16. As shown in FIG. 16, the pixel electrode 3has the stripe-shaped electrodes 8 and spaces 10 formed in a line-and-space pattern in parallel with the drain bus line 6. The stripe-shaped electrodes 8 are electrically connected by a connection electrode 64 formed in the middle of the pixel 2 substantially in parallel with a gate bus line 4. Some of the stripe-shaped electrodes 8 are connected to a source electrode 62 provided in a face-to-face relationship with a drain electrode 60 of a TFT 16.
As shown in FIG. 17, a linear protrusion 66 extending in parallel with the gate bus line 4 is formed on an opposite substrate in a position in a face-to-face relationship with the connection electrode 64 in the middle of the pixel region. The aligning direction of liquid crystal molecules 24a can be more strongly determined by the linear protrusion 66.
Obviously, a rubbing process may be performed on an alignment film on the array substrate or opposite substrate instead of providing the linear protrusion 66 on the opposite substrate. In this case, both of regions B and C of the array substrate shown in FIG. 16 are rubbed toward the connection electrode 64 as indicated by the arrows in FIG. 17. The opposite substrate is rubbed in the directions of becoming apart from the connection electrode 64. An optical method of alignment may be alternatively used.
The panel structure shown in FIGS. 16 and 17 was used to irradiate the liquid crystal layer 24 with light with the liquid crystal molecules 24a in the pixel 2 tilted in a predetermined direction by applying a voltage to the liquid crystal layer 24 added with a photo-polymeric monomer. The monomer was thus polymerized to fix the pre-tilt angle and/or alignment of the liquid crystal molecules 24a. The completed MVA-LCD was driven for display, and an observation of the display area revealed that an improvement of transmittance had been achieved over the related art in that the dark parts X1 had disappeared to allow light to be transmitted through the entire pixel regions.
In the structure proposed in the above-mentioned application, however, liquid crystal molecules located above the spaces 10 are not aligned (tilted) because they are not sandwiched by electrodes from above and below and are not directly subjected to an electric field, although the alignment of the liquid crystal layer is fixed. This results in a problem in that a reduction in transmittance occurs in the vicinity of the spaces 10. Thus, although the structure shown in FIG. 16 allows better fixation of the alignment of a liquid crystal compared to the structure shown in FIGS. 14A and 14B and improves the transmittance of a peripheral region of a pixel by preventing the occurrence of dark parts X1 as shown in FIG. 15, it has a problem in that it can not improve the transmittance of a pixel as a whole dramatically because the optical transmittance of a pixel is conversely reduced in a region inside the peripheral region.